1. Field of the Invention
The present invention relates to a nonvolatile memory semiconductor device and a method for manufacturing thereof.
2. Related Art
In the conventional technologies, a method for preventing deterioration of a gate insulating film or a tunnel insulating film by doping nitrogen to a gate insulating film of a metal oxide semiconductor (MOS) transistor or a tunnel insulating film of a nonvolatile memory semiconductor device to prevent from an electron trap is proposed (see, for example, Japanese Patent No. 3,264,265). The tunnel insulating film of the nonvolatile memory semiconductor device maintains electron in a floating gate. On the contrary, a tunneling current flows through the aforementioned tunnel insulating film in each time a writing or an erasing data is conducted. Therefore, deterioration by flowing tunneling current is caused in the tunnel insulating film of the nonvolatile memory semiconductor device. Although electron is not trapped of by the tunnel insulating film when no deterioration is caused in the tunnel insulating film as shown in FIG. 13A, a trap site for electron is created in the tunnel insulating film when a deterioration is caused in the tunnel insulating film as shown in FIG. 13B, and electron is trapped by this trap site. This provides a weakened electric field of the tunnel insulating film, leading to less flow of a tunneling current therethrough. Such deterioration of the tunnel insulating film causes a reduced frequency of writing data to the nonvolatile memory semiconductor device and erasing data stored in the nonvolatile memory semiconductor device. To solve the problem, nitrogen is doped into the tunnel insulating film, so that deterioration of the tunnel insulating film is prevented and an attempt of increasing frequency of writing data to the nonvolatile memory semiconductor device and erasing data stored in the nonvolatile memory semiconductor device is proposed (see, for example, Japanese Patent Laid-Open No. 2001-338,997). In addition to the above-described Japanese Patent Laid-Open No. 2001-338,997, another typical attempt of doping nitrogen to the tunnel insulating film is disclosed in, for example, Japanese Patent Laid-Open No. H11-317,463 (1999). Japanese Patent Laid-Open No. H11-317,463 discloses a procedure, in which a thin gate insulating film (thermal oxidation film) is formed on a silicon substrate, and a chemical vapor deposition (CVD) oxide film is deposited thereon, and then, for example, an annealing is conducted within a nitrogen atmosphere at 900 degree C. for around 10 minutes, and further, a floating gate is formed thereon.
Nevertheless, in recent years, it is a current situation that further increase in frequency of writing data on the nonvolatile memory semiconductor device and erasing thereof is required. As a result of the various experiments of the present inventors, there is still a case that the quality of the tunnel insulating film deteriorated, even if nitrogen is doped into the tunnel insulating film, and therefore has been found that it is difficult to fulfill the needs of further increasing the frequency of writing and erasing data in the technologies described in the above-described prior art documents.